The present invention relates to a hydro-statically operated continuously variable transmission including a swash-plate-type axial-piston hydraulic pump and a hydraulic motor which are coupled in a closed hydraulic circuit, the hydraulic pump and motor having respective groups of pump and motor plungers which are disposed in annular patterns in a common cylinder block that is centrally coupled to a transmission shaft.
Hydrostatically operated continuously variable transmissions are known in the art as disclosed in Japanese Laid-Open Patent Publication No. 61-153057, for example.
In the conventional hydrostatically operated continuously variable transmission, groups of pump and motor plungers are disposed perpendicularly to the axis of a cylinder block, and hence the cylinder block which accommodates these pump and motor plungers must be axially elongate. Therefore, it has been difficult to make the hydrostatically operated continuously variable transmission more compact.
A swash-plate-type hydraulic device, which may serve as the swash-plate-type axial-piston hydraulic pump or the hydraulic motor, has a working oil distributor having mutually concentric high- and low-pressure hydraulic chamber defined in the cylinder block around the axis thereof. The working oil distributor includes a number of radial distribution valves that are reciprocally movable between radially outer and inner positions in the cylinder block for bringing cylinder holes defined in the cylinder block into alternate communication with the high- and low-pressure hydraulic chambers. An eccentric ring is disposed eccentrically with respect to the center of rotation of the cylinder block in contact with the radially outward portions of the distribution rings for imparting reciprocating movement to the distribution rings in response to relative rotation of the cylinder block and the plunger swash plate. Such a working oil distributor is known from Japanese Laid-Open Patent Publication No. 61-153057.
The swash-plate-type hydraulic device tends to have an axial dimension greater than the outside diameter of the cylinder block since annular groups of plungers disposed in the cylinder holes are disposed parallel to the axis of the cylinder blocks. With the conventional working oil distributor incorporated in the swash-plate-type hydraulic device, the group of distribution valves must be located at one end of the group of plungers, resulting in an increase in the axial dimension of the cylinder block, and it is difficult to render the swash-plate-type hydraulic device more compact.